User-recordable optical recording systems typically comprise a focused laser beam for reading and recording, and an optical recording medium, such as a recordable compact disk (CD-R) or a magneto-optical disk. The medium generally includes several thin layers of material applied to a transparent substrate.
During recording, the laser beam is scanned over the recording medium while the laser power is modulated according to the input data stream. Energy absorbed from the laser causes a localized change in the physical properties of one or more layers of the medium, resulting in an optically readable mark such as a pit, bubble, or region of altered complex refractive index. Recorded information is subsequently reproduced by scanning the medium with the same laser beam at a reduced power level. For the case of a CD-R medium, the reflectivity of the written marks is decreased relative to that of the surrounding unwritten areas, causing a modulation in the reflected light intensity as the read beam scans the medium. This modulation can be converted to an electrical signal by a photodetector.
The compact disk format was originally designed as a user-read-only-memory system for use with prerecorded media in the information storage (CD-ROM) and sound recording (CD audio) fields. The prerecorded media generally have an injection molded substrate in which pits have already been formed, with a metal reflective layer deposited over the pits. These media were designed to exhibit a reflectance of about 0.75 (measured at a wavelength of 780 nm) from a flat unrecorded region of the media surface, so as to be easily readable by inexpensive CD players. The popularity of the CD format has resulted in a large number of installed CD players which are designed for operation with these highly reflective media.
The demand for user-recordable compact disk (known as CD-R) media is currently rising. An attractive feature of a CD-R medium is that, once recorded, it can be played back using any existing conventional CD audio or CD-ROM player. To ensure compatibility with existing CD players, it is desirable that CD-R media meet international specifications requiring a reflectance of .gtoreq.0.70 from a flat region of the disk, and a recording power of 4 to 8 mW. Taken together, these two requirements place considerable constraints on the design of CD-R media.
Current CD-R media constructions are typically based upon a recording layer of organic dye, which is deposited by spin coating. The recording layer is covered by a metallic reflector layer made of a material such as gold, which is generally sputter-deposited. While such constructions record satisfactorily, they have a number of associated disadvantages. For example, the manufacture of such media is relatively costly. In addition, the dye spectral response of the media is so narrow that the media may not be compatible with recorders from different manufacturers (which typically have slightly different laser wavelengths). Long term environmental and archival stability of the dye-based CD-R media is also questionable.
As an alternative to the dye-based media constructions, it is known that the necessary reflectance can generally be achieved by employing a media construction containing at least one layer of a metal such as gold, silver, copper or aluminum. However, recording directly on these metal layers requires prohibitively high laser power because of the high melting points and high thermal conductance of the metal layers. High thermal conductance causes heat from the laser to be rapidly dissipated from the recording area.